1. Field of the Invention
The present invention relates to an acoustic resonator and a method for manufacturing the same. More particularly, the present invention relates to a film bulk acoustic resonator (FBAR) using a piezoelectric material and a method for manufacturing the same.
2. Description of the Related Art
Recently there have been dramatic developments in wireless mobile communication technologies. Such mobile communication technologies require diverse radio frequency (RF) parts that can efficiently transfer information in a limited frequency bandwidth. In particular, the filter of the RF parts is one of the key elements in mobile communication technologies. This filter serves to filter innumerable waves in air to allow users to select or transfer desired signals, thereby enabling high-quality communications.
Currently, wireless communication RF filters are typically dielectric filters or surface acoustic wave (SAW) filters. A dielectric filter provides high dielectric permittivity, low insertion loss, stability at high temperatures, and is robust to vibration and shock. However, the dielectric filter cannot be sufficiently reduced in size and cannot be integrated with other integrated circuits (ICs), including the recently developed Monolithic Microwave Integration Circuit (MMIC). In contrast, a SAW filter provides a small size, facilitates processing signals, has a simplified circuit, and can be mass-produced using semiconductor processes. Further, the SAW filter provides a high side rejection in a passband compared to the dielectric filter, allowing it to transmit and receive high-quality information. However, using conventional InterDigital Transducers (IDTs) for a SAW filter limits its line width, since the process for creating such a SAW filter includes exposure to ultraviolet (UV) light. Currently, such a SAW filter's line width is limited to about 0.5 μm. Accordingly, the SAW filter cannot cover the high frequency bands, e.g., over 5 GHz. Further, it is still difficult to integrate a SAW filter with the MMIC structure on a semiconductor substrate as a single chip.
In order to overcome the limits and problems as above, a film bulk acoustic resonance (FBAR) filter has been proposed in which a frequency control circuit can be completely constructed in the form of MMIC with other active devices integrated together on an existing Si or GaAs semiconductor substrate.
The FBAR is a thin film device that is low-priced, small-sized, and can be designed to have a high-Q. Thus, the FBAR filter can be used in wireless communication equipment of various frequency bands, for example, ranging from 900 MHz to 10 GHz and military radar. The FBAR can be made an order of magnitude smaller than a dielectric filter or a lumped constant (LC) filter and has a very low insertion loss compared to the SAW filter. The FBAR can be integrated with the MMIC while providing a filter having a high stability and a high-Q factor.
The FBAR filter includes a piezoelectric dielectric material such as ZnO, AIN, or any appropriate material having a high acoustic velocity. The piezoelectric material may be directly deposited onto a Si or GaAs semiconductor substrate, e.g., by RF sputtering. The resonance of the FBAR filter arises from the piezoelectric characteristics of the piezoelectric material used therein. More particularly, the FBAR filter includes a piezoelectric film disposed between two electrodes, and generates bulk acoustic waves to induce resonance.
FIG. 1 illustrates a cross-section of a conventional membrane-based (or bulk micro-machining-based) FBAR. This membrane-based FBAR includes a silicon oxide layer (SiO2) deposited on a substrate 11 forming a membrane layer 12 on the reverse side of the substrate 11 through a cavity 16 formed by isotropic etching. A resonator 17 includes a lower electrode layer 13 formed on the membrane layer 12, a piezoelectric layer 14 on the lower electrode layer 13, and an upper electrode layer 15 on the piezoelectric layer 14.
The above membrane-based FBAR provides a low dielectric loss of the substrate 11 and less power loss due to the cavity 16. However, the membrane-based FBAR occupies a large area due to the orientation of the silicon substrate, and is easily damaged due to the low structural stability upon a subsequent packaging process, resulting in low yield. Accordingly, recently, air gap-type and Bragg reflector-based FBARs have been created to reduce the loss due to the membrane and simplify the device manufacturing process.
FIG. 2 illustrates a cross-section of a structure of a conventional air gap-type FBAR. The FBAR has a membrane layer 42 formed on a semiconductor substrate 41 and a resonator 46. The resonator 46 includes a lower electrode 43, a piezoelectric layer 44, and an upper electrode 45 deposited in that order on the membrane layer 42. The membrane layer 42 has the predetermined number of through holes 42′ used to inject etching solution therethrough. This etching solution injection forms recesses on partial portions of the substrate 41 to form air gaps 41′ between the substrate 41 and the membrane layer 42.
However, the substrate etching process for the conventional air gap-type FBAR results in many defects. Wet etching is used for the substrate etching process in general. It is difficult to remove the etching solution after the wet etching process. This residual etching solution results in minute etching. Such etching changes the resonant frequency of the resonator.
Further, when etching the substrate, the etching solution also etches part of electrodes and piezoelectric material. This unintended etching is likely to further change the resonant frequency of the resonator.
In order to insure reasonable reliability of the FBAR produced this way, an extra tuning process to set up a desired frequency is required. Further, in order to reduce the dielectric loss of the substrate, the conventional FBAR having an air gap formed through substrate etching has to use a high-resistance substrate or a very thick membrane layer, both of which are undesirable.